The invention relates to a housing for an electrical device, in particular a measuring transducer, wherein at least one electrical lead is introduced into the housing through the wall of the housing and is sealed against the wall by a sealant.
The housings of electrical devices, in particular the housings of measuring transducers, such as pressure transducers, must be adequately sealed even under large fluctuations in temperature and at high environmental humidity, to ensure long service life of the electrical device, mostly at least ten years.
The feedthroughs for electrical leads, which pass right through the housing for connection to the electrical device, must therefore also be adequately sealed. The sealant provided for that purpose must not only not let through any moisture, but must also be flexible and furthermore remain sealed even under changing temperatures. Moreover, it must not impair the conductivity and suitability for soldering of terminals of the leads lying inside the housing. The sealant could cause such an impairment by partly covering the terminals and thus reducing their contact surface for soldering on leads continuing into the inside of the device.
Two-component sealants, synthetic resins and similar substances are often used for that purpose, and a wide choice is commercially available. These sealants are not only expensive, but often also difficult to use, and are not always satisfactory, as experience has shown.
U.S. Pat. No. 4,083,902 describes a seal that consists of two discs. One disc consists of foamable adhesive, which foams at a first temperature and seals a feedthrough opening of a supply lead, thus preventing the actual sealant, consisting of a second disc that fuses at a higher temperature, from flowing away through the lead feedthrough.
According to U.S. Pat. No. 4,910,867, solder terminals are sealed by means of hot-setting adhesive, which in the first instance is placed in strip form on the terminals and is then melted so that it penetrates into the feedthrough openings between the wall thereof and the supply leads.
The methods described are complicated, however. In addition, these sealants impair the suitability of the electrical leads for soldering.
The invention is based on the problem of providing a housing of the kind mentioned in the introduction, in which sealing of the leads is simpler and more reliable.
According to the invention, this problem is solved by using tar as the sealant.
The tar can be commercial tar, such as roofing tar, which surprisingly has just the qualities desired.
An end portion of each lead projecting into the interior of the housing can be in the form of a terminal for the electrical device. The suitability of this terminal for soldering is not impaired by the sealing method using tar.